The present invention relates to a laser irradiation method of annealing a semiconductor film using a laser beam and a laser irradiation apparatus for performing the laser annealing (apparatus including a laser and an optical system for guiding a laser beam output from the laser to a member to be processed). Further, the present invention relates to a semiconductor device manufactured by the steps including the laser annealing step and a method of manufacturing the semiconductor device. Note that the semiconductor device mentioned through the specification includes an electro-optical device such as a liquid crystal display device or a light emitting device and an electronic device including the electro-optical device as its component.